Compared with previous projections, our revised base case
scenario now assumes that the 2001 and 2003 tax cuts, due to
expire by the end of 2012, remain in place. We have
changed our assumption on this because the majority of
Republicans in Congress continue to resist any measure that would
raise revenues, a position we believe Congress reinforced by
passing the act. Key macroeconomic assumptions in
the base case scenario include trend real GDP growth of 3%
and consumer price inflation near 2% annually over the
decade.

S&P downgraded the U.S., in part, because of a revised
expectation that the Bush tax cuts would remain in place. They
assumed this because of Republicans' unwillingness to enact any
measures raising revenue, and they completely slammed House
Republicans — including Paul Ryan — for doing so.

Here's a video of lines from Ryan's speech he's being most
criticized for: